1. Field of the Invention
The present invention relates to a wireless communication system, wireless communication device and wireless communication method, and computer program such as a wireless LAN (Local Area Network) wherein communication is performed mutually between multiple wireless stations, and in particular relates to a wireless communication system, wireless communication device and wireless communication method, and computer program wherein a control station adjusts access timing for various terminal stations within a network, and wherein various terminal stations are mutually synchronized to perform wireless communication in an infrastructure mode.
2. Description of the Related Art
Wireless networks are gathering attention as systems to offer freedom from cables with cabled communication methods in related art. Examples of standard specifications relating to wireless networks are IEEE (The Institute of Electrical and Electronics Engineers) 802.11 and IEEE 802.15.
A method of providing one device serving as a control station called an “access point” (AP) or “coordinator” within an area to form a network under the overall control of the control station is generally employed in order to configure a local area network using wireless technology. The control station adjusts the access timing of multiple terminal stations within the network, and performs synchronous wireless communication wherein the various terminal stations are mutually synchronized.
Also, as another method to configure a wireless network, “Ad-hoc communication” has been proposed, wherein each terminal station operates in an autonomous decentralized “Peer-to-Peer” manner, wherein the terminal stations themselves determine the access timing. For a small-scale wireless network made up of relatively few clients located near each other, ad hoc communication which can perform wireless communication asynchronously directly between terminals arbitrarily, without using a particular control station, may be appropriate.
For example, networking with IEEE 802.11 is based on the concept of BSS (Basic Service Set). BSS is made of up of two types, which are BSS wherein a control station is defined by an existing “infrastructure mode”, and an IBSS (Independent BSS) which is defined by an ad hoc mode configured only by multiple MT (Mobile Terminals).
With the infrastructure mode, the control station handles a range around itself where waves can reach as a BSS, so as to configure a “cell” in a so-called cellular system. The terminal station existing near the control station is contained by the control station, and enters the network as a BSS member. That is to say, the control station sends a control signal called a beacon at appropriate time intervals, recognizes that the terminal station which can receive this beacon exists near the control station, and further establishes a connection with the control station.
When in infrastructure mode, only the control station sends a beacon at a predetermined frame cycle. Otherwise, the periphery MT enters the network by receiving a beacon from an AP, and does not send a beacon itself.
FIG. 19 shows an operation example of IEEE 802.11 during infrastructure mode. In the example shown, the communication station STAO operates as the control station, and other communication stations STA1 and STA2 operate as terminal stations. The communication stations STAO serving as a control station sends a beacon at a fixed time interval, as shown in the chart on the right side of the diagram. The transmission point-in-time for the next beacon is managed within the control station as a parameter called TBTT (Target Beacon Transmit Time). When the point-in-time reaches TBTT, the control station operates beacon transmission procedures.
The terminal stations STA1 and STA2 in the periphery of the control station can receive the beacon alarm from the control station, and recognize the next beacon transmission point-in-time from an internal Beacon Interval field and the point-in-time of receiving the beacon. The terminal station can switch to Power Save mode (in the case receiving is not necessary) and only perform receiving operations intermittently, whereby power consumption can be reduced. Specifically, the terminal station enters a sleep state (Doze) wherein power to the receiver is lowered until the beacon receiving point-in-time (TBTT) for the next time or for multiple times to come. The timing for the various terminal stations in Power Save mode to start up (Wake) is managed in an integrated manner in the control station.
On the other hand, with the IBSS ad hoc mode, IBSS is autonomously defined after negotiating with another terminal station. The terminal station group herein fixes the TBTT as a fixed time interval. By recognizing that the TBTT has arrived by referencing a clock within the station itself, each terminal station sends a beacon, following a delay of random back-off, in the case recognition is made that no terminal station has sent a beacon.
FIG. 20 shows an operation example of IEEE 802.11 during ad hoc mode. The example in the diagram shows a state wherein two terminal stations (MT) make up an IBSS. In this case, one of the MTs belonging to the IBSS send the beacon each time a TBTT arrives. Also, there are cases wherein the beacons sent from various MTs collide. Also, with IBSS also, the MT may enter a sleep state to turn off the power of the transmitter/receiver as necessary.